Remove CHANGELOG.md

[garak]

Maintaining such a file is a pain (it involves editing it after creating a tag), and it's a duplicate of the releases page offered by GitHub. It's sometimes forgotten, so we have an inconsistency between the file and the releases page.

We should remove it. Pull requests are welcome.

[derrabus]

So this issue is basically about removing one file without replacement? If so, #483 nails it, I suppose. 🤔